Conformational analysis of methylsulphinyl derivatives of furan and thiophene by employing nuclear magnetic relaxation and lanthanide induced shifts

Abstract

Relaxation parameters such as nuclear Overhauser effect (NOE) and spin–lattice relaxation times(T₁) were applied to the conformational analysis of methylsulphinyl derivatives of furan and thiophene. Correlation times for molecular tumbling, τ_c, and for methyl reorientation, τ_i, were obtained. The τ_i values for the different molecules examined are nearly the same, the τ_c are twice as large in derivatives that also contain an iodine substituent in the heterocyclic ring. From non-selective, selective and bi-selective T₁ measurements and NOE values, relative to ¹H nuclei, and a best-fit procedure, the most probable conformational intervals for the methylsulphinyl group were located. Owing to the pyramidal structure of sulphoxides, the preferred conformers could not be fixed unambiguously. From the Lanthanide Induced Shift (LIS) technique on ¹H and ¹³C nuclei, intervals of preferred orientations of the S–O bond with respect to the heterocyclic ring were found. Reliable conclusions about the conformational behaviour of these molecules have been drawn by matching the two sets of results. Relaxation parameters turn out to be useful for conformational analysis of small molecules when employed as a complementary tool with other experimental approaches.